Document DE 42 37 879 A1 (document '879) discloses an evaluation circuit for a similarly embodied inductive sensor. Two measuring coils are switched as differential sensors, i.e., inductivities of the two coils change in opposition to one another as a function of a detected position of a core. Each measuring coil is operated together with two capacitors as a sensor oscillator that with an inverting amplifier create a digital Colpitts oscillator. The output signal of this oscillator can be immediately further processed in a digital evaluation circuit. The oscillation duration of the oscillator is determined in that an oscillator signal releases a counter and after, for instance, two oscillation periods, the counter is stopped. The counter reading of the counter increases synchronously with a cycle frequency that is supplied by a separate oscillator and that is substantially higher than the frequency of the sensor oscillator. Then, the counter readings for the two opposing sensor oscillators are subtracted and output as digital measurement results.
Known from document DE 40 38 515 A1 (document '515) is another device for measuring longitudinal or angular position. It is used for instance in motor vehicles to monitor a vehicle body level or to measure an angular position of a crankshaft. Problems associated with the measuring device are a large temperature range and other negative effects caused by ambient conditions. A sampling coil is part of an LC resonant circuit, which is itself operated by a controlled sampling oscillator. As an analog circuit, oscillator circuit and resonant circuit produce an alternating voltage, the frequency of which is determined by the resonant circuit but is not substantially changed during the measurement. On the contrary, a plurality of sampling oscillators are used adjacent to one another, and the amplitudes of their resonant circuits are influenced by damping and non-damping of a plurality of parallel code tracks. The resonant circuit is tuned to somewhat different resonance frequencies in order to eliminate interfering cross-talk between the sampling coils. For damping each sampling coil, a binary track is used that has been calibrated in the longitudinal unit and that comprises surfaces that are alternately conducting and non-conducting metals. In order to obtain reliable measurement results over a wide temperature range, an additional reference oscillator is provided that is arranged in a manner largely identical to that of the sampling oscillators in terms of structure and space such that it is subjected to the same ambient conditions as the sampling oscillators. The reference oscillator always remains undamped; in addition it is controlled analogously in a mode with a constant amplitude. Its controlled variable, which reflects environmental factors, is used for controlling the amplitude of the actual sampling oscillators and thus as a comparison value for voltages for the sampling oscillators.